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Chapter 1 Electricity Section 2 Electric Current, adopted from Glencoe Science Modules: Physical Science, Electricity and Magnetism, Student Edition Section 2 Electric Current Flow of Charge As You Read What You'll Learn Relate voltage to the electric energy carried by an electric current. Describe a battery and how it produces an electric current. Explain electrical resistance. Vocabulary electric current circuit voltage resistance Why It’s Important The electric appliances you use rely on electric current. An electric discharge, such as a lightning bolt, can release a huge amount of energy in an instant. However, electric lights, refrigerators, TVs, and stereos need a steady source of electric energy that can be controlled. This source of electric energy comes from an electric current, which is the flow of charge through a conductor. In solids, the flowing charges are electrons. In liquids, the flowing charges are ions, which can be positively or negatively charged. Electric current is measured in units of amperes (A). A model for electric current is flowing water. Water flows downhill because a gravitational force acts on it. Similarly, electrons flow because an electric force acts on them. A Model for a Simple Circuit How does a flow of water provide energy? If the water is separated from Earth by using a pump, the higher water now has gravitational potential energy, as shown in Figure 10. As the water falls and does work on the waterwheel, the water loses potential energy and the waterwheel gains kinetic energy. For the water to flow continuously, it must flow through a closed loop. Electric current will flow continuously only through a closed conducting loop called a circuit. Figure 10 Water can acquire energy when a pump separates the water from Earth. The greater the height is, the more energy the water has. What can water with energy do? Sect2ElectCurrent Electricity and Magnetism Page 1 Chapter 1 Electricity Section 2 Electric Current, adopted from Glencoe Science Modules: Physical Science, Electricity and Magnetism, Student Edition Figure 11 A battery causes electrons to move from the negative to the positive battery terminal. As the electrons move, they lose energy. Why do electrons have less energy after passing through the lightbulb? How a Current Transfers Energy Just as the flow of water can do work in a waterwheel, current flow can do work in an electric device, such as a lightbulb. As current flows, it carries electrical energy through the wire. When the current passes through the wire in the lightbulb, the electrons in the bulb wire lose some of this energy, as shown in Figure 11. This energy is converted by the lightbulb to radiant and thermal energy. Voltage-Electric Potential In a water circuit, water gains potential energy by being pumped from a lower level to a higher level. The pump provides this energy. In an electric circuit, a battery is a source of electric potential energy. In the water circuit, the distance the pump lifts the water is a measure of the available potential energy of the water. Likewise, the voltage of a battery is a measure of how much electric energy it can provide. Voltage is measured in volts (V). A typical flash-light battery is 1.5 V. How a Current Flows You may think that when an electric current flows in a circuit, electrons are traveling completely around the circuit. Actually, individual electrons move slowly through a wire in an electric circuit. When the ends of the wire are connected to a battery, electrons in the wire begin to move toward the positive battery terminal. As an electron moves it collides with other electric charges in the wire, and is deflected in a different direction. After each collision the electron again starts moving toward the positive terminal. A single electron may undergo more than ten million million collisions each second. As a result, it may take three hours for an electron in the wire to travel one meter. Potential in a Battery A cutaway view of an alkaline battery is shown in Figure 12. A battery has two terminals-a negative terminal and a positive terminal. How does a battery produce electric potential energy? In batteries, the electric potential energy comes from chemical energy. At the negative terminal, a chemical reaction converts metal atoms into ions, releasing electrons. At the positive terminal, another reaction converts metal ions of a different metal into metal atoms by accepting the electrons. These reactions create an electric potential difference between the two battery terminals. When the battery is connected to a circuit, this electric potential energy cause an electric current to flow from the negative terminal to the positive terminal. The energy Sect2ElectCurrent Electricity and Magnetism Page 2 Chapter 1 Electricity Section 2 Electric Current, adopted from Glencoe Science Modules: Physical Science, Electricity and Magnetism, Student Edition carried by the circuit can then be used to light a flashlight or run a radio. The amount of electric potential energy-or voltage-that a battery has depends on the amount and type of chemicals used in the battery. Battery Life Batteries don't supply power forever. Maybe you know someone whose car wouldn't start after the lights have been left on overnight. Why do batteries run down? Batteries contain only a limited amount of the chemicals that react to produce chemical energy. These reactions go on as the battery is used, and the chemicals are changed into other compounds. Once the original chemicals are used up, the chemical reactions stop, and the battery is dead. Many chemicals are used to make an alkaline battery. Zinc is a source of electrons and positive ions, manganese dioxide is used to collect the electrons at the positive terminal, and water is used to carry ions through the battery. Visit the Glencoe Science Web site at science.glencoe.com for information about the chemistry of batteries. Figure 12 The chemical reactions in an alkaline battery create a difference in electric potential energy between the positive and negative terminals of the battery. Resistance Electrons can move much more easily through conductors than through insulators, but even conductors interfere some-what with the flow of electrons. The measure of how difficult it is for electrons to flow through a material is called resistance. The unit of resistance is the ohm (Ω). Insulators generally have much higher resistance than conductors. Sect2ElectCurrent Electricity and Magnetism Page 3 Chapter 1 Electricity Section 2 Electric Current, adopted from Glencoe Science Modules: Physical Science, Electricity and Magnetism, Student Edition As electrons flow through a circuit, they collide with the atoms and other electric charges in the materials that make up the circuit. Look at Figure 13. These collisions cause some of the electrons' electric energy to be converted into thermal energy-heat-and sometimes into light. The amount of electric energy that is converted into heat and light depends on the resistance of the materials in the circuit. Wires and Filaments The amount of electric energy that is converted into thermal energy increases as the resistance of the wire increases. Copper, which is one of the best electric conductors, has low resistance. Copper is used in household wiring because little electric energy is lost as current flows through cop-per wires. This means that not much heat is produced. Because copper wires don't heat up much, the wires don't become hot enough to melt through their insulation, which makes fires less likely to occur. On the other hand, tungsten wire has a higher resistance. As current flows through tungsten wire, it becomes extremely hot-so hot, in fact, that it glows with a bright light. The high temperature makes tungsten a poor choice for house-hold wiring, but the light it gives off makes it an excellent choice for the filaments of light bulbs. Reading Check Is having resistance in electrical wires ever beneficial? Figure 13 As electrons flow through a wire, they travel in a zigzag path as they collide with atoms and other electrons. These collisions cause the electrons to lose some electric energy. Where does this electric energy go? Figure 14 For water and electrons, the diameter and length of the conductor influence resistance. A A narrow hose increases the resistance. B A long hose also increases the resistance. Slowing the Flow Think of water. What causes the flow of water through a hose to slow down? A narrow hose would slow the flow. Would a longer hose achieve the same thing? Figure 14 shows two examples of how the resistance of a water hose can be increased. Likewise, the length and diameter of a wire affects electron flow. A short, thick wire has less resistance than a long, thin wire, and it is a better conductor. Sect2ElectCurrent Electricity and Magnetism Page 4 Chapter 1 Electricity Section 2 Electric Current, adopted from Glencoe Science Modules: Physical Science, Electricity and Magnetism, Student Edition Section 2 Assessment 1. How does increasing the voltage in a circuit affect the energy of the electrons flowing in the circuit? 2. What causes positive and negative charges to be separated in a battery? 3. For the same length, which has more resistance-a garden hose or a fire hose? Which has more resistance-a thin wire or a thick wire? 4. Why is copper often used in household wiring? 5. Think Critically Some electrical devices require two batteries, usually placed end to end. How does the voltage of the combination compare with the voltage of a single battery? Try it. 6. Drawing Conclusions Observe the size of various batteries, such as a watch battery, a camera battery, a flashlight battery, and an automobile battery. Conclude whether the voltage produced by a battery is related to its physical size. For more help, refer to the Science Skill Handbook. 7. Communicating The terms circuit, current, and resistance are often used in everyday language. In your Science Journal, record several different ways of using the words circuit, cur-rent, and resistance. Compare and contrast the everyday use of the words with their scientific definitions. For more help, refer to the Science Skill Handbook. Sect2ElectCurrent Electricity and Magnetism Page 5